The alternative pathway of complement is a phylogenetically ancient arm of the innate immune system that eliminates invasive pathogens and facilitates the removal of injured host cells. See J. M. Thurman et al., J. Immunol. (2006) 176:1305-1310. The alternative pathway is continually auto-activated in the fluid phase, forming C3b which can bind to nearby biologic surfaces. This spontaneously formed C3b then catalyzes further activation and amplification through the alternative complement pathway unless controlled by complement regulatory proteins (CRPs). The CRPs dissociate the alternative pathway C3 convertase (C3bBb) and/or serve as cofactors for the cleavage of C3 by factor I, forming iC3b. Thus, complement inhibition by CRPs on host cells is critical for protecting host cells from spontaneous alternative complement pathway-mediated injury. Expression of CRPs is a fundamental mechanism by which the alternative pathway distinguishes healthy cells from injured cells and invasive pathogens.
The endogenous membrane-bound proteins that control alternative pathway activation are decay-accelerating factor (DAF/CD55), membrane cofactor protein (MCP/CD46), and complement receptor 1 (CR1). Other endogenous proteins that control alternative pathway activation include Factor H, a circulating ˜155 kD glycoprotein that regulates alternative pathway activation in the fluid phase as well as on tissue surfaces. See J. J. Alexander et al., Mol. Immunol. (2006) 44:123-132. Uncontrolled alternative pathway activation has been implicated in the pathogenesis of a diverse group of diseases, including age-related macular degeneration (AMD), atypical hemolytic uremic syndrome (aHUS), type II membranoproliferative glomerulonephritis (MPGN II), asthma, and renal ischemia/reperfusion (I/R) injury. See J. M. Thurman et al., J. Immunol. (2006) 176:1305-1310. Injury to host tissues by the alternative pathway indicates insufficient local control of the alternative pathway by the target tissue. Indeed, recent studies have demonstrated that mutations in CRPs are strong risk factors for aHUS (M. C. Pickering et al., J. Exp. Med. (2007) 204:1249-1256) and MPGN II (R. J. Smith et al., J. Am. Soc. Nephrol. (2007) 18:2447-2456), and functional polymorphisms in factor H, a circulating regulator of the alternative pathway, are associated with the development of AMD (R. J. Klein et al., Science (2005) 308:385-389; A. O. Edwards et al., Science (2005) 308:421-424; J. L. Haines et al., Science (2005) 308:419-421; G. S. Hageman et al., Proc. Nat'l Acad. Sci. USA (2005) 102:7227-7232).
Ischemic acute kidney injury (AKI) in rodents (J. M. Thurman et al., J. Immunol. (2003) 170:1517-1523; J. M. Thurman et al., J. Am. Soc. Nephrol. (2006) 17:707-715) and in humans (J. M. Thurman et al., Kidney Int. (2005) 67:524-530) is associated with activation of the alternative pathway on the basolateral surface of injured tubular cells. We have found that Complement receptor 1-related gene/protein y (Crry, a rodent analog of human MCP and CR1) is the only CRP expressed by proximal tubular epithelial cells in mice, and that ischemia/reperfusion causes reduced surface expression of this protein. See J. M. Thurman et al., J. Clin. Invest. (2006) 116:357-368. Mice with congenital deficiency of Crry (Crry+/−) are more sensitive than wild-type controls to ischemic acute renal failure (Id.), highlighting the importance of basolateral Crry for controlling the alternative pathway on this surface. It is not yet known whether polymorphisms or mutations in the CRPs may confer increased risk of developing AKI in humans. Nevertheless, uncontrolled activation of the alternative pathway in the setting of reduced surface Crry indicates that circulating factor H has a limited ability to protect the surface of hypoxic tubular epithelial cells.
Factor H circulates in high concentrations (>400-600 μg/ml) and is a potent inhibitor of the alternative complement pathway. See J. J. Alexander et al., Mol. Immunol. (2006) 44:123-132. Alternative pathway inhibition on cell surfaces by factor H, however, requires that it properly bind to that surface. Several regions within the factor H protein bind to anionic surfaces, such as membranes rich in heparin sulfate or sialic acid, as well as to C3b on the surface. See S. Meri et al., Proc. Nat'l Acad. Sci. USA (1990) 87:3982-3986; M. K. Pangburn et al., J. Immunol. (2000) 164:4742-4751. Activation of the alternative pathway on a particular surface is strongly influenced by the affinity of factor H for that surface. The polymorphisms and mutations associated with AMD and aHUS, respectively, most frequently involve the region of factor H required for binding anionic surfaces and not the complement regulatory region. See M. C. Pickering et al., J. Exp. Med. (2007) 204:1249-1256; A. P. Sjöberg et al., J. Biol. Chem. (2007) 282:10894-10900. Thus, certain tissues or cell types require factor H to regulate alternative pathway activation on their surface. Different binding regions of the factor H protein may be necessary for complement regulation on those tissues or cell types. In some cases, the binding of factor H to surfaces in particular tissues may be affected by other proteins. Identification of putative tissue-specific binding partners of factor H may provide potential mechanisms for modulating, i.e., stimulating or inhibiting, activity of the alternative complement pathway in different tissues.
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